1. Field of the Invention
The present invention relates to a sheet feeding apparatus for separating and feeding a sheet sheet by sheet and to an image forming apparatus such as a copying machine, a printer or the like or an image reading apparatus having the apparatus.
2. Related Background Art
Conventionally, for example, in a sheet feeding section of a copying machine or the like, a sheet separation using a retard roller rotating in a direction opposite to a sheet feeding direction has mainly used as a sheet feeding means for preventing a plurality of sheets from being fed (referred to as xe2x80x9cdouble-feedxe2x80x9d hereinafter).
A summary of a conventional sheet feeding apparatus utilizing a retard separation system will be described hereinafter.
FIG. 25 is a schematic side view of a sheet feeding apparatus utilizing a retard separation method constituted of a sheet feeding roller and a separation roller (see, Japanese Unexamined Patent Publication (KOKAI) Heisei No. 3-18532, U.S. Pat. No. 5,016,866). This is referred to as a first prior art hereinafter.
First, as shown in FIG. 25, sheets S stacked on a intermediate plate 206 in a cassette 207 are lifted together with the intermediate plate 206 by a pressing arm 208 and a sheet pressing spring 205 to be always pressingly contacted with the sheet feeding roller 201, thereby gaining sheet feeding pressure.
Also, the sheet feeding roller 201 receives retard pressure (separation roller pressure) from a separation roller 202. In this state, when the sheet feeding roller 201 rotates in a sheet feeding direction, the sheets S pressingly contacted with the sheet feeding roller 201 are fed out to reach a nip formed by the sheet feeding roller 201 and the separation roller 202. At this time, when a single sheet S is pinched by the nip, the separation roller 202 is also rotationally driven in the sheet feeding direction together with the sheet feeding roller 201 due to a torque limiter 203 formed integrally with a separation roller shaft, thereby feeding the sheet S.
When a plurality of the sheets are pinched in the nip, however, the separation roller 202 is rotated with predetermined torque in a direction to which the double-fed sheets are restored, with the operation of the torque limiter 203, thereby preventing the sheet double-feed.
In addition, FIG. 26 and FIG. 27 are schematic side views of a sheet feeding apparatus utilizing a retard separation system with a planetary gear mechanism (see, Japanese Patent Publication (KOKOKU) Heisei No. 1-32134). This is referred to as a second prior art hereinafter.
As shown in FIG. 26, this sheet feeding mechanism utilizes a planetary gear mechanism constituted of a sun gear 301, an intermediate gear 302, a planetary gear 303 and a connecting arm 304, and a sheet feeding roller 307 is connected to the planetary gear 303. Furthermore, a separation roller 309 is connected to a driving shaft 306 through a torque limiter, and a pair of draw rollers 310 for conveying a sheet S at a speed higher than a speed at which the sheet feeding roller 307 feeds the sheet S is disposed on a downstream of the sheet feeding roller 307 in a sheet feeding direction.
An operation of the sheet feeding mechanism will be briefly described hereinafter with reference to FIG. 27.
First, by rotation of the driving shaft 306, the planetary gear 303 and the sheet feeding roller 307 revolve in a direction indicated by an arrow A in FIG. 27, with the result that the sheet feeding roller 307 pressingly contacts with an uppermost sheet S of the sheets stacked within a sheet cassette. Also, in synchronous with the rotation, a lever 318 lifts an intermediate plate 323 stacking the sheets toward a direction of the sheet feeding roller (a direction indicated by an arrow G in FIG. 27).
By this operation, the sheet S pressingly contacted with the sheet feeding roller 307 is sent into a nip formed by the sheet feeding roller 307 and the separation roller 309, thereby feeding and separating the sheet S. Furthermore, the sheet S after passing through the nip enters into the pair of draw rollers 310, and the planetary gear mechanism and the sheet feeding roller 307 are returned to the initial positions by transmitting the driving force of the pair of draw rollers 310 through the sheet S to the planetary gear mechanism, and this operation is repeated.
Although the two prior arts in the sheet feeding mechanism are shown, it is considered that the respective prior arts can be improved in several points.
First, in the mechanism of the first prior art, the sheets S stacked on the intermediate plate 206 in the cassette 207 are lifted together with the intermediate plate 206 to be always pressed by the sheet pressing spring 205. Consequently, a sheet feed and separation condition greatly depends on the pressure of the intermediate plate, resulting in that an appropriate sheet feed area is limited in consideration of the pressure of the intermediate plate as a function.
In particular, because the pressures of the intermediate plate generated by the sheet pressing spring 205 vary with the number of sheets stacked in the cassette 207, the sheet feed and separation condition differs between a case where the sheets are fully loaded and a case where a several number of the sheets are loaded. Also, the pressure of the intermediate plate is always generated on the stacked sheets S since the sheets S are always pressingly contacted with the sheet feeding roller 201. Therefore, while the sheet S as the uppermost sheet is being fed, the sheet Sxe2x80x2 to be succeedingly fed is subjected to a conveying force by friction between the sheets, with the result that the double-feed of the sheet Sxe2x80x2 tends to easily occur.
In addition, even if the double-fed sheets are separated and tried to be returned to the former position, the sheet S is pinched by the sheet feeding roller 201 and the intermediate plate 206, and therefore the double-fed sheet may not be returned smoothly.
Furthermore, an allowable range of the appropriate sheet feed area is limited in dependence upon the kind of sheet (for example, sheet having great coefficient of the friction) or the reduction of frictional coefficients of the sheet feeding roller and the separation roller caused by wears, and as a result, the stability might be worsened. Therefore, it is hard to say that this mechanism is a sheet feeding mechanism having high stability and high reliability.
Incidentally, in this mechanism, when it is tried that the double-feed is hard to occur and the double-fed sheets can easily restored, the restoring force provided by the torque limiter 203 must be set to a greater value; the retard pressure of the retard spring must be considerably decreased; or the pressure of the intermediate plate provided by the sheet pressing spring 205 must be considerably decreased.
In any cases, however, slip between the sheet feeding roller 201 and the separation roller may be generated, with the result that the wear of the sheet feeding roller 201 and the separation roller 202 is accelerated, thereby considerably reducing enduring lifetime of the sheet feeding roller 201 and the separation roller 202. As a result, the number of periodical replacing operation for worn parts is increased to increase the maintenance cost of the sheet feeding apparatus. Furthermore, torque of the driving force applying means (motor) is required to increase, resulting that not only the cost of the apparatus but also power consumption must be increased.
In addition, when the restoring force of the torque limiter 203 is set to be a greater value, in a space Z formed between a nip portion X constituted of the sheet feeding roller 201 and the separation roller 202, and a pressingly contacting part Y between the sheet feeding roller 201 and the sheets on the intermediate plate 206, it is considered that the double-fed sheet (especially, thin sheet having poor elasticity) may be buckled, thereby causing sheet jam.
Also, when a pair of conveying rollers are provided on a downstream side of the sheet feeding roller 201 and the separation roller 202 in the conveying direction, it is considered that the pair of conveying rollers must draw the sheet, which is continuously under pressure, from the intermediate plate 206 as well as the nip between the sheet feeding roller 201 and the separation roller 202, with the result that greater load will act on the pair of conveying roller, thereby shortening the enduring lifetime of the conveying rollers.
Furthermore, since the intermediate plate 206 is always pressed by the sheet feeding roller 201 by means of the sheet pressing spring 205, if this prior art is applied to a multi-feeding section, a user, when setting sheets, must push the intermediate plate 206 down against the sheet pressing spring 205 to create a gap between the intermediate plate 206 and the sheet feeding roller 201, thus to insert the sheets into the gap.
Consequently, it is hard to say this apparatus has good operability, and as a result, the user easily fails in setting the sheets properly, which may, in turn, cause sheet jam or skew feed.
Next, in the mechanism of the second prior art, the sheet feeding roller 307 carries out pressing and estranging operations with respect to the stacked sheets S, and accompanying this operations, the intermediate plate 323 is also moved up and down by the lever 318, thereby effecting the pressing and the pressure releasing operations. In other words, when the sheets S stacked on the intermediate plate 323 are fed, the sheets S are in a state of being pinched by the sheet feeding roller 307 and the intermediate plate 323 from the upside and the downside.
Furthermore, the estranging operation of the sheet feeding roller 307 and the lowering operation of the lever 318 are effected by utilizing conveying force when the conveyed sheet S is pinched by the pair of draw rollers 310. Accordingly, the sheet feeding roller 307 and the intermediate plate 323 are in a state of pinching the stacked sheets S until a leading end of the conveyed sheet S reaches the nip between the pair of draw rollers 310.
Since the sheet feeding roller 307 pressingly contacts with the sheets S during the separating operation, the sheet are hard to be separated, and furthermore, since the leading end of the sheet S reaches the nip between the pair of draw rollers 310 while the sheet feeding roller 307 is under the pressingly contact operation, there is no timing for restoring the double-fed sheets.
In consideration of the sheet feeding and separation condition, the sheet feeding mechanism according to the second prior art is the same as the sheet feeding mechanism according to the first prior art. Thus, as is in the first prior art, in this mechanism, the appropriate sheet feed area is narrow, so it is hard to say that it has high stability and high reliability. In addition, the structure is very complicated and a large number of the parts are required.
Further, because the pressure releasing operation of the sheet feeding roller 307 with respect to the sheets S and the revolving operations of the planetary gear mechanism and the sheet feeding roller 307 are effected by the conveying force of the pair of draw rollers 310, great conveying load acts on the draw rollers 310, and it is, therefore, considered that the enduring lifetime of the draw rollers 310 becomes shortened.
As a problem common to the above two prior arts, there is raised a problem that stability and reliability of the sheet feeding and separation operations are not fully secured since the pressure of the intermediate plate affects the sheet feeding and separation condition. Further, there is raised as another problem that the double-feed easily occurs during the separating operation since the sheets stacked on the intermediate plate are pressingly contacted with the sheet feeding roller; there is no timing for restoring the double-fed sheets; and then, in dependence upon the kind of sheet, the sheet may be buckled to cause sheet jam.
In consideration of the above-mentioned problems, it is an object of the present invention to provide a sheet feeding apparatus which accomplishes both high enduring property and high separating property with a simple structure to achieve improving stability and reliability as a sheet feeding apparatus, reduces maintenance cost of the apparatus, and accomplishes both the cost reduction and the apparatus miniaturization due to simplification of the structure.
In order to accomplish the above objects, it is a representative structure according to the present invention to include sheet supporting means for supporting sheets, feeding means pressingly contacting with the sheet supported by the sheet supporting means to rotate in a direction of conveying the sheets for feeding the sheet, a separation rotator pressingly contacting with the feeding means to rotate in a direction to which the sheet is restored for separating the sheets fed from the feeding means to be a piece, drive transmitting means for driving the feeding means, and pressure switching means for switching pressure of the separation roller with respect to the feed means.